What is Alzheimer’s disease?

Alzheimer’s is a form of dementia that affects people at an advanced age.

People with Alzheimer’s develop problems with thinking, memory and as the disease progresses, it can affect the daily activities and tasks carried.

What led Scientists to Believe That There Could be a Genetic Basis to Alzheimer’s?

Scientists found that if a father, mother, brother or sister had Alzheimer’s, then the likelihood of an individual getting Alzheimer’s was greater. This led them to investigate and to study the effect of gene mutations that could increase the risk for the disease.

Scientists discovered certain genes that are linked to Alzheimer’s but the presence of gene mutations at these sites increase the risk for Alzheimer’s but doesn’t necessarily lead to the development of Alzheimer’s.

Genetic Testing for Alzheimer’s disease The various genes associated with Alzheimer’s disease that are determined after independent research studies, need to be validated further. Moreover, the genes that are popular in specific populations need to be studied to understand the importance of specific genes in that particular population.

Genetic testing for Alzheimer’s disease is not a routine, however whenever there is a strong family history some adults may wish to know their chances of getting the disease later in life. Any testing is not fool proof and even if you test positive it is possible that you may escape the affliction due to other factors too. Generally it has been found that an active healthy life that includes a diet rich in fruits and vegetables with physical exercise and keeping mental faculties alert through cross-words or Sudoku will delay the onset of the disease.

Familial Alzheimer’s disease (FAD) is a rare form of early onset Alzheimer’s disease that is passed down the family. It is inherited from a parent and accounts for 2-3% of all Alzheimer’s disease, it is in this form that genetic testing can be undertaken routinely.

Prenatal Testing will indicate if the unborn baby has an elevated risk of developing AD. However, if the parents are unwilling to terminate the pregnancy even if there is high risk, the testing need not be carried out.

Pediatric Testing: Children can be tested for the presence of high risk gene mutations. However, it is not advised as Alzheimer’s is a condition that will probably occur only after many years for children and it is unwise for parents or the child to worry about it from now.

Genetic Testing for Adults: Symptomatic people or adults who wish to ascertain their disease risk can get themselves tested for the presence of high risk alleles.

Genetic Counseling: People with the high-risk allele can be counseled about the disease and the methods of care. These sessions will be useful for caregivers too.

What are the Types of Techniques that are Used to Identify Genes Associated with Alzheimer’s?There are four methods that are used to identify genes that are associated with Alzheimer’s.

Linkage Analysis: Genetic linkage analysis is used to identify the chromosomal region that is associated with an increased risk for Alzheimer’s but it will not be able to identify a specific gene or a specific mutation that is associated with the disease condition.

2. PSEN1:

Present on the chromosome 14q24.3

50% of EOAD show mutations in PSEN1

Mutation in PSEN1 leads to:

30% loss of neuronal function in the hippocampus CA1 region.

18% hippocampus atrophy

Leads to neurodegeneration.

3. PSEN 2:

Present on chromosome 1q31-q42

People with this mutation have a wide range of onset - 39 to 75 years

What are the Genes Associated with Late-Onset Alzheimer’s Disease (LOAD)?LOAD is more complex that EOAD as it has multiple genes along with environmental factors that influence the development of the disease condition.

Genes that Influence Cholesterol MetabolismThe association between cholesterol metabolism genes and the onset of Alzheimer’s disease showed that the accumulation of large amount of cholesterol in the body during middle age increases the risk for Alzheimer’s Disease (AD) during later stage in life.

1. APOE gene:

Present on chromosome 19q13.2

Strong risk factor for LOAD

Mutations in the gene have been associated with:

Increased hippocampal atrophy

Decline in gray matter as age advances

Elevated amyloid load

Impairment of glucose metabolism

Cerebral amyloid angiopathy

Increase in microbleeds

2. CLU gene:

Present on chromosome 8p21-p12

An important risk factor in AD

Apolipoproteins are expressed in the brain and in the periphery

Extracellular chaperone in lipid transport

Sperm maturation

Apoptosis (programmed cell death)

Endocrine secretion

Complement rejection

Membrane protection

Patients with AD show elevated levels of clusterin (CLU) gene in the hippocampus, frontal cortex and the cerebrospinal fluid (CSF).

Increased plasma concentrations of CLU leads to:

Atrophy of the brain

Increases disease severity

Clinical progression of people affected with AD

3. ABCA7:

Present on chromosome 19p13.3

Associated with LOAD

It mediates

Transport of high density lipoprotein

Lipids are effluxed from cell into lipoprotein membrane

ABCA7 is associated with AD through various pathways that include phagocytosis, lipid metabolism and Aβ accumulation.

4. SORL1:

Present on chromosome 11q23.2-q24.2

Lower expression of SORL1 leads to an increase in production of Aβ, which is associated with AD

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